1. Field of the Invention
This invention pertains to heat treatment. More particularly, this invention pertains to heat treatment of metallic parts utilizing direct infrared radiation as the heat source.
2. Description of the Prior Art
The present invention describes an improved apparatus and method to the teachings of commonly assigned U.S. Pat. No. 5,306,359. That patent teaches an apparatus and method for heat treating metallic parts with direct infrared radiation as the heat source. As described in the '359 patent, the preferred embodiment utilizes the apparatus and method for heat treating aluminum or aluminum alloy parts such as automobile or truck wheels.
As disclosed in U.S. Pat. No. 5,306,359, the heat treatment of cast or forged aluminum parts requires heating the part to a desired temperature and then rapidly cooling the part. For example, an aluminum part may need to be heat treated to about 1,020.degree. F. and then rapidly cooled or quenched by immersing the part in water or other quenching fluid. After quenching, the part is reheated to about 300-500.degree. F. in an aging stage and held at the aging temperature for a period of time.
As disclosed in U.S. Pat. No. 5,306,359, historical heat treatment techniques for aluminum alloys were very time consuming and capital intensive. Also, as disclosed in that patent, the historical techniques for heat treating parts such as aluminum alloy wheels perform the heat treatment in a batch process. For example, a plurality of aluminum castings or forgings are placed on a pallet or other device in a common oven and heat treated or aged as a collective group. Accordingly, there may be variations among the various parts of the batch. As a result, certain parts in the batch may not be suitably heat treated and may be subject to rejection. Also, during the quenching stage, the entire batch is placed in a quenched liquid. Since the parts are commonly stacked one upon the other, the quenching fluid cannot adequately surround the entire surface of each part in the batch. Further, due to the fact that the parts are batch treated, there is no unique metallurgical history as to the specific heat treatment of each part.
In U.S. Pat. No. 5,306,359, each part is uniquely heat treated in a series of heat treatment stations. Each of the heat treatment stations includes infrared radiation lamps for directly heating the part with infrared radiation. A part is sequentially moved from station to station. As shown best in FIGS. 1 and 2 of the '359 patent, the part is heated while rotating the part within a station to assure uniform heating. Infrared radiation lamps do not completely surround the part. Instead, the lamps are positioned on an upper surface and two opposing sidewalls of the heat treatment station. To permit passage of a part from one station to another, the stations do not have end walls separating the stations. As a result, infrared radiation lamps are not positioned on any end walls and, therefore, there are no infrared lamps surrounding the part on all sides.
As disclosed in U.S. Pat. No. 5,306,359, it is important to determine the actual temperature of a part prior to the part being admitted to any subsequent heat treatment station. The temperature is used by a controller of the device in order to independently control the infrared radiation lamps within any given heat treatment station. Namely, by knowing the temperature of the part admitted to a heat treatment station, the controller can calculate the amount of intensity required by the lamps in the particular station such that the heat treating within that station and within any anticipated subsequent heat treatment station will elevate the part to a desired final temperature.
As indicated in U.S. Pat. No. 5,306,359, measurement of the actual temperature of an aluminum alloy part is difficult. The most precise way to measure such a temperature is to implant a thermocouple within the part. However, this is not practical in a process intended for the mass heat treatment of a large number of parts. Accordingly, U.S. Pat. No. 5,306,359 utilizes optical pyrometers or other non-contact, infrared sensors to measure the actual temperature of a part.
Aluminum is extremely non-emissive. The low emissivity of aluminum makes temperature measurement with optical pyrometers difficult. In U.S. Pat. No. 5,306,359, a method for calculating the actual temperature of the part is disclosed where an optical pyrometer is aimed at the part and a separate optical pyrometer is aimed within the heat treatment chamber to measure background radiation. Further, a thermocouple is placed within the refractory wall of the chamber. The combination of readings from the optical pyrometer measuring off of the part, the optical pyrometer measuring background radiation within the heat treatment station and the thermocouple measuring the temperature of the refractory are applied to an empirical equation to determine an apparent actual temperature of the part. In practice, the lamps used to heat the part remain emissive and may interfere with the actual measurement of the temperature of a particular part.
It is an object of the present invention to provide an apparatus and method for uniquely heating a plurality of metallic parts with an improved apparatus and method for both heating the parts and for measuring the actual temperature of a part at any given location within the apparatus.